1. Field of the Invention
The present invention relates to a power converter, and in particular to a power converter including an overload protection function known as a hiccup mode.
2. Description of the Related Art
In a flyback converter, the input and output sides of the circuit are isolated from one another via the primary and secondary coils of a transformer. To ensure reliable operation of the converter, a control signal indicating the voltage or current condition of the transformer is passed from the output side of the converter control circuit to the input side, so that the switching of the primary and second transformer coils can be properly regulated. Converter control circuits typically operate according to voltage mode control or current mode control, depending on the type of control signal that is used, a voltage sense signal or a current sense signal, for example.
Both control methods require a signal related to the output voltage of the transformer to be fed back to the converter control circuit. There are two common ways to feedback an output sense voltage. First, coupling the secondary side circuit (the output stage) of the transformer to the control circuit using an opto-coupler, or alternately, using a separate winding (an auxiliary transformer) on the transformer coil, and relying on the cross regulation of the design to feed back an appropriate voltage signal to the control circuit. Opto-couplers provide tight voltage and current regulation properties, but are more complex and costly to install than auxiliary transformers. Auxiliary transformers on the other hand, while not providing as good regulation as opto-couplers, are less costly and tend to be more reliable in long term operation.
The purpose of the converter control circuit is to regulate the switching or duty cycle of the primary transformer coil to achieve a suitable and predicted voltage output at the secondary circuit or output stage, as well as to ensure safe operation. Some Pulse Width Modulator (PWM) power supply controllers provide what is known as “hiccup mode” short circuit protection against overloads. Hiccup-mode operation of a power supply protects the power supply from damage due to an over-current fault condition. It also allows the power supply to restart when the fault is rectified.
Hiccup mode may be triggered when the control circuit senses a reduction in the output sense voltage of the transformer. For example, if a reduction in output voltage to say 60% of the expected value is detected, the control circuit interprets this as an overload condition. The control circuit shuts off the power supply for a given time and then attempts to restart the power supply again. If the overload condition has been removed, the power supply will start up and operate normally; otherwise, the controller will see another overload event and shut off the power supply again, repeating the previous cycle. By repeatedly switching off and restarting the converter circuit, the hiccup mode results in greatly reduced power loss.
In a primary side regulated converter, where voltage feedback is provided by an auxiliary winding, hiccup mode of operation is not however possible because imperfect cross regulation prevents the voltage on the feedback winding from reducing when the output load is high and the auxiliary load is low. It has been known to try and solve this problem by using overload protection methods such as: fuses, PTCs (Positive Temperature Coefficient) sensor and thermal trips, secondary side feedback using opto-isolated feedback, or direct connection (non-isolated), and secondary side current limit circuits. However, these methods are costly and can lead to complex circuit designs.
Thus, it would be desirable to provide an improved converter in which overload conditions can be readily detected, allowing reliable triggering of hiccup mode.